Manufacture of white cast iron

ABSTRACT

An improved method for the production of martensitic and/or lower bainitic white cast iron. The method of this invention includes the steps of preparing a melt of molten cast iron having a chromium content in the range of 0.5 percent to 15 percent, a total additional content of hardenability increasing alloying elements in the range of trace to 3 percent, casting into a mold having a maximum cavity thickness of about 2.5 inches and retaining the casting in the mold until it cools to a shake-out temperature in the range of 1,700*F. to 1,950*F.. The casting is then cooled after shake-out in still air to a temperature in the range of 1,325*F. to 1,650*F. and thereafter the casting is quenched when at a temperature within the range of 1,325*F. to 1,650*F. by immersing it in the liquid quenching medium at a temperature above 80*F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting the casting.

United States Patent [191 Davies [451 Jan. 8, 1974 MANUFACTURE OF WHITE CAST IRON [75] Inventor: Barry John Davies, Burlington,

Ontario, Canada [22] Filed: Sept. 29, 1972 [21] Appl. No.: 293,339

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 93,754, Nov. 30,

1970, abandoned.

[52] U.S. Cl. 148/3, 148/138, 148/141 [51] Int. Cl C21d 5/04, 322d 25/00 [58] Field oi Search 198/2, 3, 13, 14, 198/138, 141, 35

[56] References Cited UNITED STATES PATENTS 2,192,645 3/1940 Lanenstein et al. 148/141 X 2,352,408 6/1944 Reece et al. 148/3 3,022,205 2/1962 Chase et al. 148/206 OTHER PUBLICATIONS Physical and Engineering Properties of Cast iron, Angus, British Cast Iron Res. Assoc. 1960, page 363-367, 370,371 & 373-378.

Metals Handbook, 8th Ed. Vol. 2, 1964, pages 204-207, 209 & 212.

Primary Examiner-Charles N. Lovell AttorneyGordon W. Hodson et a1.

[57] ABSTRACT An improved method for the production of martensitic and/or lower bainitic white cast iron. The method of this invention includes the steps of preparing a melt of molten cast iron having a chromium content in the range of 0.5 percent to 15 percent, a total additional content of hardenability increasing alloying elements in the range of trace to 3 percent, casting into a mold having a maximum cavity thickness of about 2.5 inches and retaining the casting in the mold until it cools to a shake-out temperature in the range of 1,700F. to 1,950F.. The casting is then cooled after shake-out in still air to a temperature in the range of 1,325F. to 1,650F. and thereafter the casting is quenched when at a temperature within the range of 1,325F. to 1,650F. by immersing it in the liquid quenching medium at a temperature above 80F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting he casting.

6 Claims, 1 Drawing Figure (AIUCAL TEMPERA TURE RANG' STA/P7 0F TRANSFORM/1 T/OIV 6ND 0F fRAA/SFORMA T/ON PEA lPl/I'' flPPER BA lN/TE MARTA-Mil?! PATENIEDJAH 8 m4 3784416 C/P/T/CAL TEMPERA TURE RANGE START OF TRANSFORMATION 'A/D 0F IRA MSFORMA r/o/v TEMPERATURE 1014/5? BAl/V/TE MARTEMS/T' CROSS REFERENCES This application is a continuation-in-part of application Ser. No. 93,754, filed Nov. 30, 1970 now abandoned.

This invention relates to the manufacture of white cast iron. In particular the invention provides an improved method of producing white martensitic and/or lower bainitic cast iron with a relatively low content of alloying elements which increase the hardenability.

PRIOR ART Martensitic and/or lower bainitic white cast iron has high wear resistant and abrasion resistant characteristics and, as a result, martensitic and/or lower bainitic white cast iron is used in the manufacture of a wide range of castings wherein these characteristics are required. The martensitic and/or lower bainitic structure in white cast iron is achieved in the known processes by employing a relatively high content of hardenability increasing alloys. The increase in hardenability obtained by adding certain alloying elements to the iron results from delaying the transformation times of the higher temperature transformation products such as pearlite and bainite. Many of the hardenability increasing alloys, such as nickel, chromium, molybdenum, copper and vanadium are expensive alloys and the cost ofthese alloy additions is a substantial factor in the cost of producing martensitic and/or lower bainitic white cast iron. in the known method of producing martensitic and/r lower bainitic white cast iron, the hardenability increasing alloys are added in the melt, in the furnace or in the receiving or pouring ladle. Molten iron of white iron composition is poured into a sand mold or into a permanent mold such as a cast iron or graphite mold. After pouring, the casting is allowed to solidify in the mold and, at some time after the casting has solidified, it is removed from the mold. When a permanent mold is used, the conventional practice is to shake out the casting from the mold shortly after solidification. An important step in most of the present methods for the production of martensitic and/or lower bainitic white cast iron is the step of retarding the rate of cooling of the casting. The rate of cooling can be retarded by retaining the casting in the sand mold or if it is produced in a permanent mold, the casting may be placed into a sand pile immediately after shake-out. Depending upon the geometry of the casting, it has been known to reduce the rate of cooling by means of a soaking pit or the like in order to prevent cracking. in some instances, the castings are allowed to cool in air and where the geometry of the casting is very simple, cooling may take place in moving air. No methods are presently in use in the production of martensitic and/or lower bainitic white cast iron wherein the casting is quenched, immediately after shake-out at a ternperature above the critical temperature, into a quenching media so as to effect a very rapid cooling of the casting. In fact, in most processes for producing martensitic and/or lower bainitic white cast iron, great care is taken to prevent rapid cooling of the casting when it is shaken out at temperatures above the transformation temperature. It has been generally believed that rapid cooling of the casting from a high shake-out temperature to below the Ms temperature (temperature of start of transformation of austenite to martensite) sets up stresses in the casting which are very undesirable in a material of this extremely brittle nature and may cause cracking.

SUMMARY The present invention provides; an improved method of producing martensitic and/or lower bainitic white cast iron. According to an embodiment of the present invention, martensitic and/or lower bainitic white cast iron having a chromium content in the range of 0.5 percent to 15 percent is produced by a method which in cludes the steps of preparing a melt of molten cast iron having a hardenability increasing alloy content consisting of from 0.5 percent to 15 percent chromium and from trace to 3 percent of a material selected from the group consisting of nickel, molybdenum, copper, manganese and vanadium, casting the molten cast iron into a mold having a maximum cavity section thickness of about 2.5 inches, retaining the casting in the mold until it cools to a shake-out temperature in the range of 1,700F. to 1,950F., cooling the casting after shakeout in still air to a temperature in the range of 1,325F. to 1,650F. and thereafter quenching the casting when at a temperature in the range of 1,325F. to 1,650F. by immersing it in a liquid quenching medium selected from the group consisting of; aqueous solutions of organic polymers, polyvinyl alcohol, molten salt and quenching oils, at a temperature above F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting the casting. As a result of the fact that this method permits a casting to be quenched shortly after shake-out, it is possible to produce white martensitic and/or lower bainitic cast irons without requiring a holding furnace for elevated temperature treatments or the reheating furnace which is used in the known methods. In the production of white cast iron, this method has been shown to produce a crack-free martensitic and/or lower bainitic white cast iron casting from a melt having a relatively low content of the more expensive hardenability increasing alloys. In addition, it has been found that the martensitic and/or lower bainitic white cast iron produced according to the method of the present invention not only requires lower alloy content at lower cost, but also results in improved structural and physical properties compared to conventionally produced irons. It has been found that the structure of the cast iron produced by the method of this invention is superior to conventionally produced irons because only a small amount of austenite is retained in the cast product. As a result, the hardness of the martensitic and/or lower bainitic white cast iron produced according to the method of the present invention is higher than that of many of the martensitic and/or lower bainitic white cast irons obtained by the methods of the prior art. The improvements in the art described above were totally unexpected having regard to the fact that it was widely believed that a martensitic and/or lower bainitic white cast iron casting could not be produced with a low content of hardenability increasing alloy without excessive cracking occurring as a result of quenching.

The invention will be more clearly understood with reference to the following examples and the accompanying drawing wherein,

FIG. 1 is a schematic illustration of the temperaturetime-transformation curve for the low alloy martensitic and/or lower bainitic cast iron disclosed herein.

With reference to FIG. 1 of the drawings, this curve applies to several types of irons with different alloy contents, no absolute times or temperatures are listed. The curve B represents a typical cooling curve for an iron cooled in air in the conventional manner. It will be noted that this curve intersects the low alloy transformation curve in the area of the pearlite nose (the shortest time for pearlite transformation) and, as a result, pearlite is formed in the material. Pearlite has previously been shown to be undesirable as it detracts from the required qualities of high wear and abrasion resistance in cast iron. By adding a predetermined amount of nickel or other hardenability increasing alloys, it is possible to move the transformation curve to the right of the graph which has the effect of increasing the time period provided before transformation occurs. Under these conditions the conventional cooling in air (curve B) would not pass through the pearlite nose" and, consequently, no pearlite would be formed in the iron cooled in the conventional manner.

Curve A illustrates a typical cooling curve for iron cooled according to the present invention. This cooling rate can be achieved by quenching in a liquid medium immediately after shake-out at a temperature above the critical temperature. As shown in FIG. 1, this cooling rate has the effect of cooling an iron casting of the lower alloy content sufficiently rapidly to prevent the formation of pearlite.

An important step in the production of white cast iron according to the present invention is in the holding of the casting in the mold until it cools to a shake-out temperature in the range of l,'700F. to 1,950F. After shake-out, the casting is cooled in still air to a temperature in the range of l,325F. to l,650F. prior to quenching. The purpose of retaining the casting in the mold to the shake-out temperature specified above and the step of cooling the casting after shake-out in still air to a temperature in the range specified is to minimize existing thermal gradients thereby reducing the probability of cracking during subsequent quenching. Because of the cracking which occurs during quenching, it is necessary, even with the present invention, to limit the maximum sectional thickness of the casting to about 2.5 inches. Even with cooling in still air, it is not possible to eliminate all of the existing thermal gradients with the result that sections greater than two and one-half inches are very susceptible to cracking during quenching.

It is essential that the cooling rate be accelerated while the casting is still above the critical temperature. Therefore, the castings must be cooled in the mold to a shake-out temperature in the range of l,700F. to 1,950F., removed from the mold, cooled in still air to a temperature in the range of l,325F. to 1,650F., and quenched while still above the critical temperature.

When a casting at a temperature in the range of 1,325F. to l,650F. is placed into a selected liquid medium, a blanket of vapour immediately envelops the material. The manner in which this vapour forms bubbles and thereby removes itself from the surface of the casting affects the uniformity of heat extraction from various areas of the casting surface. Since it is desirable to remove heat from the surface of the casting in as uniform manner as possible in order to minimize the stresses incurred in the casting during cooling, the cooling or quenchant medium should be selected to provide properties which allow a uniform distribution of heat extraction.

The liquid quenchant should also exhibit a moderating influence on the rate of heat extraction. Although a very rapid rate of heat extraction is desirable from the point of view of minimizing the amount of alloying elements necessary to prevent the formation of pearlite, an extremely rapid rate of cooling will also cause the formation of a considerable temperature gradient between the surface and the centre of the casting. A steep temperature gradient in a material of the brittle nature of white cast iron may cause cracking.

If the material is cooled sufficiently rapidly to prevent the formation of pearlite (which forms at temperatures between approximately l,325F. and 850F.), the original austenite will transform to martensite and/or lower bainite at lower temperatures. This transformation occurs extremely rapidly and is accompanied by a volume increase in the material. If the casting is cooled rapidly through the temperature at which martensite begins to form, the outside of the casting, being at a lower temperature, will transform more rapidly than the inside. Under these circumstances the outside of the casting would become brittle and when the centre section is transformed with an accompanying volume increase, the outside layer would be stressed and cracking could occur. This can be prevented by holding the casting isothermally at a temperature just above the Ms temperature and then removing the casting from the bath and allowing it to cool slowly in air. An effective media for this type of treatment is a bath of molten salt.

An alternative method is to time the length of quench in the liquid medium such that the casting cools quickly from the critical temperature to a temperature just above that at which the austenite transforms to martensite. The casting is then withdrawn from the bath and allowed to cool in air. The liquid medium employed may be various oils, or water base solutions, or emulsions containing selected additional agents that delay the rate of cooling. A water soluble organic polymer identified by the Trade Mark UCON QUEN- CHANT A can be mixed with water to provide the required quenching medium.

When producing 1 1/2 inches diameter white cast iron spheres, water base solutions containing 15 percent, 20 percent and 25 percent Ucon A by volume of an organic polymer were found to cool the castings sufficiently slowly to prevent the cracking of the castings. Dwell times in the quenching tank of 40 seconds were adequate to prevent the formation of pearlite. Castings were left in the bath until they attained the bath temperature 1 10F.) and did not exhibit any signs of cracking. An alloy containing 0.6 percent nickel and 0.6 percent manganese, when quenched in Ucon A of the indicated concentrations, exhibited no signs of pearlite.

The following Examples will serve to illustrate that martensitic and/or lower bainitic white cast iron can be successfully produced without requiring a high percentage content of hardenability increasing alloys in the casting.

EXAMP E 1 T.C. S Si Mn Cr Ni This iron was cast into cast iron permanent molds to make castings in the shape of a truncated cone, the end diameters of which were l-r inches and 1 inch respectively, and the height of which was linches. The castings were shaken out of the molds at approximately l,800F. and were allowed to cool in still air to a temperature of about l,550F. The castings were then immersed into a water base quench bath containing a water soluble polymer. The castings were allowed to cool to a bath temperature of about 100F.

After quenching, the structure contained approximately 50 percent carbides and 50 percent retained austenite. The hardness was Re 55.

After tempering for four hours at 600F., the structure contained approximately 50 percent carbides, 35 percent retained austenite and percent martensite and/or lower bainite. After tempering, the castings exhibited a hardness of Re 57.5.

. EXAMPLE. 2

This iron was cast into green sand molds to make 1 /2 inch diameter spheres. The castings were shaken out of the molds at about 1,700F. and cooled in still air to about 1,650F. and were then immersed into a bath of molten salt held at a temperature of 500F.. The castings were allowed to remain in the salt bath forfive minutes and were then withdrawn and allowed to cool in still air. After tempering at 525F. for four hours, the structure of the material contained approximately 50 percent carbides, 5 percent retained austenite and 45 percent martensite and/or lower bainite.

Careful examination revealed no quench cracking.

EXAMPLE. 3

This iron was cast into green sand molds to make 1 /2 inch diameter spheres. The castings were shaken out of the molds at about 1,700F. and cooled in still air to about 1,650F. and were then immersed in a bath of a water base quenchant containing a water soluble polymer. The castings were allowed to cool to the bath temperature (80F).

After quenching, the structure contained approximately 50 percent carbides, percent retained austenite and 30 percent martensite and/or lower bainite. The hardness was Re 57.4.

After tempering for four hours at 525F., the structure contained approximately 50 percent carbides, 5

percent retained austenite and 45 percent martensite and/or lower bainite. After tempering, the castings exhibited a hardness of RC 61.0.

No quench cracks were observed.

E AMPLE 4.

A melt containing a conventional martensitic and/or lower bainitic white iron analysis of the following composition was prepared:

This iron was cast into green sand molds to make 1 /2 inch diameter spheres. The castings were shaken out of the molds at about 1,700F. and were allowed to cool in air. The structure of the as-cast product contained approximately 50 percent carbides, 25 percent retained austenite and 25 percent martensite and/or lower bainite. The as-cast structure exhibited a hardness of RC 54.2.

After tempering at 525F. for four hours, the structure contained approximately 50 percent carbides, 15 percent retained austenite and 35 percent martensite and/or lower bainite. The hardness of the tempered product was RC 59.6.

EXAMPLE 5 Cr Ni 5 2.0 2.0 0. l 2

This iron was cast into cast iron permanent molds to make castings in the shape of a truncated cone, the end diameters of which were 3 inches and 2- /2 inches respectively, and the height of which were 3 inches. The castings were shaken out of the molds at about 1,850F. and were then allowed to cool in still air to a temperature of about 1,550F. A casting was then quenched by means of a fine spray of quenchant containing 10 percent organic polymer in aqueous solution. This casting exhibited a large number of quench cracks.

The castings produced by conventional treatment and composition of Example 4 were compared with the material produced in Example 3 under conditions of repeated light load impact. All castings were tempered at 525F. for four hours before testing.

The quenched material outlined in Example 3 withstood percent more blows than the conventional material of Example 4 before spalling or exfoliation occurred. The quenched material withstood approximately 10 percent more blows before major fracture occurred.

Metallographic inspection of the 0.7 percent Ni quenched material revealed that a slightly larger proportion of the matrix transformed to martensite and/or lower bainite in the as-cast condition than occurred in the conventional alloyed white iron.

After tempering for four hours at 5 25F the 0.7 percent Ni quenched material contained less than 5% retained austenite in the matrix whereas the conventional alloyed white iron generally contained 15 percent 20 percent by volume of retained austenite.

Although the as-cast hardnesses of the quenched material and the conventional alloyed white iron material did not differ greatly, the hardness of the quenched material was considerably higher than that of the conventional alloyed white iron after tempering at 525F. for four hours and, as a result, the quenched material exhibits better wear or abrasion resistance than the conventional alloyed white iron material.

From the foregoing it will be apparent that the present invention provides an improved method for the production of martensitic white cast iron. The improvements in the art described above were totally unexpected, having regard to the fact that it was widely believed that a martensitic and/or lower bainitic white cast iron could not be produced with a low content of hardenability increasing alloy without excessive cracking occurring as a result of quenching.

What I claim as my invention is:

1. A method producing a martensitic and/or lower bainitic white cast iron having a chromium content in the range of 0.5 percent to percent and a maximum section thickness of about 2 /2 inches comprising the steps of,

a. preparing a melt of a molten cast iron having a hardenability increasing alloy content consisting of from 0.5 percent to 15 percent chromium and from trace to 3 percent of a material selected from the group consisting of nickel, molybdenum, copper, manganese and vanadium,

b. casting the molten cast iron into a mold having a maximum cavity section thickness of about 2.5 inches,

0. retaining the casting in the mold until it cools to a shake-out temperature in the range of l,700F. to l,950F.,

d. cooling the casting after shake-out in still air to a temperature in the range of 1,325F. to l,650F. and thereafter quenching the casting when at a temperature in the range of l,325F. to l,650F. by immersing it in a bath of liquid quenching medium selected from the group consisting of aqueous solutions of organic polymers, polyvinyl alcohol, molten salt and quenching oils at a temperature above 80F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting the casting.

2. A method producing a martensitic and/or lower bainitic low chromium white cast iron having a chromium content in the range of 0.5 percent to 3 percent and a maximum section thickness of about Z-Vz inches comprising the steps of,

a. preparing a melt of a molten cast iron having a hardenability increasing alloy content consisting of from 0.5 percent to 3 percent chromium and from trace to 3 percent of a material selected from the group consisting of nickel, molybdenum, copper, manganese and vanadium,

b. casting the molten cast iron into a mold having a maximum cavity section thickness of about 2.5

inches,

0. retaining the casting in the mold until it cools to a shake-out temperature in the range of 1,700F. to l,950F.,

d. cooling the casting after shake-out in still air to a temperature in the range of 1,325F. to l,650F. and thereafter quenching the casting when at a temperature in the range of 1,325F. to l,650F. by immersing it in a bath liquid quenching medium selected from the group consisting of aqueous solutions of organic polymers, polyvinyl alcohol, salt bath and conventional quenching oils at a temperature above F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting the casting.

3. A method producing a martensitic and/or lower bainitic high chromium white cast iron having a chr0- mium content in the range of 8 percent to 15 percent and a maxium section thickness of about 2-/; inches comprising the steps of,

a. preparing a melt of a molten cast iron having a hardenability increasing alloy content consisting of from 8 percent to 15 percent chromium and from trace to 3 percent of a material selected from the group consisting of nickel, molybdenum, copper, manganese and vanadium,

b. casting the molten cast iron into a mold having a maximum cavity section thickness of about 2.5 inches,

0. retaining the casting in the mold until it cools to a shake-out temperature in the range of l,700F. to l,950F.,

d. cooling the casting after shake-out in still air to a temperature in the range of 1,325F. to 1,650F. and thereafter quenching the casting when at a temperature in the range of 1,325F. to 1,650F. by immersing it in a bath liquid quenching medium selected from the group consisting of aqueous solutions of organic polymers, polyvinyl alcohol, molten salt and conventional quenching oils at a temperature above 80F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting the casting.

4. A method as claimed in claim 1 wherein the casting is cooled by quenching to a temperature below the Ms temperature.

5. A method as claimed in claim 1 wherein the casting is cooled by quenching to a temperature above the Ms temperature and thereafter cooled in air.

6. A method as claimed in claim 1 wherein the casting is quenched in a bath of molten salt held isothermally at a temperature not lower than the Ms temperature and not more than 300F. above the Ms temperature. 

2. A method producing a martensitic and/or lower bainitic low chromium white cast iron having a chromium content in the range of 0.5 percent to 3 percent and a maximum section thickness of about 2- 1/2 inches comprising the steps of, a. preparing a melt of a molten cast iron having a hardenability increasing alloy content consisting of from 0.5 percent to 3 percent chromium and from trace to 3 percent of a material selected from the group consisting of nickel, molybdenum, copper, manganese and vanadium, b. casting the molten cast iron into a mold having a maximum cavity section thickness of about 2.5 inches, c. retaining the casting in the mold until it cools to a shake-out temperature in the range of 1,700*F. to 1,950*F., d. cooling the casting after shake-out in still air to a temperature in the range of 1,325*F. to 1,650*F. and thereafter quenching the casting when at a temperature in the range of 1, 325*F. to 1,650*F. by immersing it in a bath liquid quenching medium selected from the group consisting of aqueous solutions of organic polymers, polyvinyl alcohol, salt bath and conventional quenching oils at a temperature above 80*F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting the casting.
 3. A method producing a martensitic and/or lower bainitic high chromium white cast iron having a chromium content in the range of 8 percent to 15 percent and a maxium section thickness of about 2- 1/2 inches comprising the steps of, a. preparing a melt of a molten cast iron having a hardenability increasing alloy content consisting of from 8 percent to 15 percent chromium and from trace to 3 percent of a material selected from the group consisting of nickel, molybdenum, copper, manganese and vanadium, b. casting the molten cast iron into a mold having a maximum cavity section thickness of about 2.5 inches, c. retaining the casting in the mold until it cools to a shake-out temperature in the range of 1,700*F. to 1,950*F., d. cooling the casting after shake-out in still air to a temperature in the range of 1,325*F. to 1,650*F. and thereafter quenching the casting when at a temperature in the range of 1, 325*F. to 1,650*F. by immersing it in a bath liquid quenching medium selected from the group consisting of aqueous solutions of organic polymers, polyvinyl alcohol, molten salt and conventional quenching oils at a temperature above 80*F. so as to cool the casting at a rate of cooling sufficient to prevent the formation of pearlite without cracking or distorting the casting.
 4. A method as claimed in claim 1 wherein the casting is cooled by quenching to a temperature below the Ms temperature.
 5. A method as claimed in claim 1 wherein the casting is cooled by quenching to a temperature above the Ms temperature and thereafter cooled in air.
 6. A method as claimed in claim 1 wherein the casting is quenched in a bath of molten salt held isothermally at a temperature not lower than the Ms temperature and not more than 300*F. above the Ms temperature. 